AD-AO93 092 NYDRONAUTICS INC LAUREL NO F/G 13/2-7AN ANALYSIS OF THE BERWICK BAY BRIDGE PASSAGE. CU)G
JUN So P V DYKE DOT-CG-8140165-A
UNCLASSIFIED TR-7909-4 USCG-D-68-80 NL
Reor0. CG-D-68-84Q
AN ANALYSIS
OF THE
BERWICK BAY BRIDGE
PASSAGE
PETER VAN DYKEDTIC
~EL F~r -
C
Document is available to the public through theNational Technical Information Service,
Springfield, Virginia 22151
FINAL REPORT
JUNE 1980Prepared for
U.S. DEPARTMENT OF TRANSPORTATIONUnited States Coast Guard
Office of Research and DevelopmentWashing-ton, D.C. 206W0
80 12 19 037:
- - mom--
NOTICE
This dowummnt Is dissenmina under the sponsorship of the Departntof Transportation I the Interest of Information exchang. Th UnitedState Glovenient swame no liability for Its contents or use thereof
The contents of this reor do not nincemaIlv reflet the official viewor policy of the Coast Guard; and the do not constitute a s tan-dudspecification, or regulation.
This report, or portions theref maey not be used for advertising orsalfs promnotion purposes. Citation of trade nam and manufacturersdoes not constitute mndorhmnt or approval of such products.
1. RportNO.Technical Report Documentation Peg.t
1. eprt .. 2.Gove'nMen, Accession No. 3. Recipient's Cetslg N.
CG--68-80 -4. ___________
.Title end Sqjbtwle 5. ReotDate
June 19806. Performing Organization Cede
An Analysis of the Berwick Bay Bridge Passage
7. Au8os . Performing Organzation Report No.
Peter Van Dyke 7909-4~9. Performing O'sw .&at.:# Name end Address UD. Walk Unmit No. 4TRAIS)Hydronautis, Inc.7210 Pindell School Rd. I1I. Contract or Grant No.
Laurel, Md. 20810
12. Sponsoring Agency Hoe, end Address13 TyeoRprtadP idCvrd
U. S. Department of TransportationU. S. Coast GuardOffice of Research and Development 14 pneigAgency Cede
Washington, D. C. 20593 G-DMT-1/5415. Supplementary Nots
The CG technical representative for this work was David A. Walden
16. I~~t
A towboat simulator analysis of the Atchafalaya River at Berwick,La., has been carried out. Autopilot and manned passages were made onthe simulator with different tow characteristics, going both upriverand downriver, and with and without bow thruster assistance. Fivedifferent power levels were represented by different engine RPM valuesand this combined with the two different tow lengths results in tenHP/L ratios for analysis. In addition, a casualty analysis was madeof an April 1, 1978 collision between a tow and the bridges over theriver.
Ke 11 11. Distributien Statement
rwc*ByBridges Navigation Available to public through:Casualty Analysis PilotingInland Waterways Towboat National Technical InformationManeuvering PoweringSevc
TowboatSevcSimulator Springfield VA 2161
It. Ssrinty Clee;si f. (of lts repert) 3D. Somirrity Clesal. (of this page) 1.N.of Paes 22. Prie
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HYDRONAUTICS, Incorporated
TECH1NICAL REPORT 7909-4
AN .kNALYSIS OF THE
-,ERWICK JAY/BffG FAMAGE v
Peter Van/Dyke/
/)June 1980
NTIS A&rDTIC TA
Prepared for : 1utfCAliLUnited States Coast Guard B
Washington, D. C. itiut~f/ -
Contract No 7D,-C-4# - Avlilit Codes
DIst Speclai.
TABLE OF CONTENTS
Page
INTRODUCTION............................................ 1
DESCRIPTION OF THE USCG TOWBOAT MANEUVERINGSIMULATOR............................................. 3
THE BERWICK BAY BRIDGE PASSAGE......................... 11
FAST-TIME AUTOPILOT RUNS................................ 19
REAL-TIME OPERATOR RUNS................................. 53
M/V STUD ACCIDENT SIMULATIONS.......................... 73
CONCLUSIONS............................................. 83
REFERENCES............................................. 89
LIST OF FIGURES
Page
FIGURE 1 - Control Console of Towboat Simulator ....... 4
FIGURE 2 - Simulator Computer System ................. 5
FIGURE 3 - Plan Position Display on SimulationControl Console ........................... 6
FIGURE 4 - Computer Generated Visual Scene, River
Tow Passing Under Bridge .................. 7
FIGURE 5 - Sample Autopilot Tracks ................... 10
FIGURE 6 - Chart of Berwick Bay ...................... 12
FIGURE 7 - Berwick Bay Passage ....................... 13
FIGURE 8 - Operator Strategy ......................... 14
FIGURE 9 - Berwick Bay Current Map ................... 17
FIGURE 10 - First Autopilot Runs ...................... 24
FIGURE 11 - Results of First Autopilot Runs ........... 36
FIGURE 12 - Second Set of Autopilot Runs, TENNESSEE .,. 39
FIGURE 13 - Second Set of Autopilot Runs, NASHVILLE 40
FIGURE 14 - RMS Track, Path and Heading Errors FromSecond Set of Autopilot Runs .............. 42
FIGURE 15 - Track, Path and Heading Errors for
TENNESSEE, Second Set of Autopilot Runs ... 43
FIGURE 16 - Track, Path and Heading Errors for
NASHVILLE, Second Set of Autopilot Runs ... 44
FIGURE 17 - Upriver Autopilot Tracks .................. 45
FIGURE 18 - Upriver Autopilot Runs .................... 46
FIGURE 19 - Results of Upriver Autopilot Runs ......... 48
FIGURE 20 - Downriver Autopilot Runs with Bow Thruster. 49
-iii-
Page
FIGURE 21 - Results of Autopilot Runs with BowThruster, Downriver ........................ 50
FIGURE 22 - Upriver Autopilot Runs with Bow Thruster 51
FIGURE 23 - Results of Autopilot Runs with BowThruster, Upriver .......................... 52
FIGURE 24- Real-Time Simulation, TENNESSE, Run 2 ...... 57
FIGURE 25 - Real-Time Simulation, TENNESSEE, Run 1,Track Following ............................ 58
FIGURE 26 - Real-Time Simulation, NASHVILLE, Run 1Bow Thruster ............................... 59
FIGURE 27 - Real-Time Simulation, NASHVILLE, Run 2,Track Following ................................. 60
FIGURE 28 - Real-Time Simulation, NASHVILLE, Run 1,Upriver With Bow Thruster .................. 61
FIGURE 29 - Real-Time Simulation, NASHVILLE, Run 1,Upriver Track Following .................... 62
FIGURE 30 - Real-Time Simulation, NASHVILLE, Run 6,Downriver, 157 RPM ......................... 63
FIGURE 31 - Real-Time Simulation, NASHVILLE, Run 5,157 RPM Track Following .................... 64
FIGURE 32 - RMS Track and Swept Path Error WhileTrack Following ............................... 65
FIGURE 33 - RMS Heading Error While Track Following .... 66
FIGURE 34 - Path, Track and Heading Errors atRailroad Bridge .......... . 9. ... 68
FIGURE 35 - Histogram of Path Error at Railroad Bridgefor Downriver Runs With and Without BowThruster . ................................. . , 72
FIGURE 36 - Autopilot Tracks for M/V STUD CasualtyAnalysis .......... . . .. .......... ,.., 74
-iv-
Page
FIGURE 37 - M/V STUD Run 4, Recovery Started atPoint C, 100 RPM ........................ 77
FIGURE 38 - M/V STUD Run 13, Recovery Started atPoint B, 100 RPM ........................ 78
FIGURE 39 - M/V STUD Run 9, Recovery Started atPoint A, 150 RPM ........................ 79
FIGURE 40 - M/V STUD Run 1, Recovery Started atPoint A, 100 RPM ........................ 80
FIGURE 41 - M/V STUD Simulation Results, Swept Pathsat Bridges .............................. 81
LIST OF TABLES
TABLE I - Manned Simulation Runs........................55
TABLE 2 -M/V STUD Runs.................................76
INTRODUCTION
The United States Coast Guard (USCG) Towboat Maneuvering
Simulator was designed, built and tested under a contract with
HYDRONAUTICS, Incorporated. The objective of the second phase
of the work under this contract has been to demonstrate typical
uses of the simulator system in areas of USCG interest involving
evaluation of vessel maneuverability, navigation rules and casu-
alty analysis. This objective was to be reached by conducting
a series of simulations for a selected river location. The
location selected was the portion of the Atchafalaya River at
Morgan City/Berwick, Louisiana. In this area the river forms
a bend and then passes under two highway bridges and one railroad
bridge with a lift span. These bridges, and in particular the
railroad bridge, are subjected to numerous rammings by tows, especially
during periods of high water which result in fast down-river
currents.
This section of the Atchafalaya River was selected for this
study since, because of the numerous casualties there, considerable
data exist on operating conditions and current velocities.Further, the USCG operates a Vessel Traffic Service (VTS) there
which involves navigation rules with respect to vessel size as
a function of river stage (current velocity), The simulation
studies were directed at an evaluation of the effects of current,
tow size, towboat horsepower, wind and the use of bow thrusters
on path error during the passage of the bridges. These results
can then be used to develop a further understanding of casualty
situations, the effects of environmental conditions and a general
correlation with the VTS navigation rules.
The first step in this study was to obtain a precise des-
cription of the current speed and direction in this section of
-MAP-. ..
-2-
the river. This was obtained from a computer river flow model
run at the Waterways Experiment Station (WES) of the U.S. Army
Corps of Engineers at Vicksburg, Mississippi under USCG sponsor-
ship. This current data was then integrated into the simulation.
* An initial series of fast-time runs under the control of a track-
following autopilot then were carried out. These runs involved
variation of current velocity, tow size, towboat horsepower and
wind direction. The results were used to narrow in on several
specific cases for more detailed study. These cases were then
investigated by more fast-time autopilot runs and by real-time
runs using an experienced river pilot, an experienced towboat
simulator user and a less experienced simulator user.
During the course of the study, a detailed National Trans-
portation Safety Board (NTSB) Marine Accident Report concerning
the April 1, 1978 collision between the M/V STUD and the railroad
bridge was received. Based on this report, further analysis of
the casualty was carried out. This analysis, which involved
combined autopilot and real-time runs, investigated the effect of
towboat power on the ability to maneuver out of an out-of-shape
condition.
The following sections of this report present a description
of the USCG towboat maneuvering simulator, a description of the
Berwick Bay Bridge Passage, the results of the various simulation
runs and conclusions.
wi
-3-
DESCRIPTION OF THE USCG TOWBOAT MANEUVERING SIMULATOR
The Towboat Maneuvering Simulator consists of a mathematical
description of the hydrodynamic response of an integrated river
tow embodied in a computer program running continuously on a
computer, with a control console and graphic and hard-copy devicesattached for input and output. It is thus a real-time, interactivesimulator, constantly responding to console commands and immediately
updating the console displays. It can be used also in an auto-pilot mode, whereby the console is superceded by a mathematical
control algorithm which calculates the rudder and throttle commands
at every time step based on a prescribed path and desired RPM
history.
The control console is shown in Figure 1, with the controlsand indicators labeled. Figure 2 shows the controlling computerand hard-copy print device. Figure 3 shows the plan position
display which is updated at specified time intervals on the graphicsimulation control console.
The computer-generated visual scene shown in Figure 4 is
a new addition to the simulator not included in the original
descriptive reports (References 1-3). It is a perspective view
of the scene from a specified viewing point, with a variable scale
and perspective factor. For the Berwick Bridge simulations, theeye position was specified at the rear of the tow at a height of30 feet above the water, looking out horizontally. The view
approaching the bridges in Figure 4 shows the bow of the tow,
the bridge piers on the west side of the highway bridge passage,with the bridges running above, the railroad bridge lift span,
and the remainder of the railroad bridge at its 10 foot elevation
above the water. The far bank beyond the bridges can be seenin the railroad span openings and above the rest of the railroad
bridge.
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The visual scene was an important addition to the simulator
for the manned runs. For the initial turn down the river (see
Figure 3), the left (east) bank is shown clearly on the screen,
and is used to judge distance from the bank. The intermediate
passage toward the bridges is then carried out primarily referring
to the plan position display on the graphic screen with the new
tow position being drawn every 20 seconds. As the passage through
the bridges nears, primary attention shifts to the visual scene,
updating every 4 seconds, where alignment of the tow with the
openings is immediately judged, and turn rates are apparent in
the relative motions between the tow and the vertical bridge
piers and lift span supports.
An autopilot to be used in fast-time runs through the bridge
passage also has been developed subsequent to the original simu-
lator version. This autopilot calculates a command steering rudder
angle based on four parameters: instantaneous distance from and
angle deviation from a prescribed track; instantaneous turning
rate; and a time averaged value of previous command rudder values.
The desired tracks are of two types: straight lines and circular
arcs. Straight line segments are defined by an X and Y coordinate
of the line origin, a length of the line segment, and its angular
position. Circular arc sections are defined by the X and Y
coordinates of the arc center, the radius of the arc, and the
angle at which the arc ends. Positive and negative radii denote
clockwise and counter-clockwise progression around the arc.
Four coefficients, a function of a particular vessel, are
needed to determine the four components of the rudder position
calculation. Distance off-track is the perpendicular distance
from the track divided by the vessel length. The heading term
is the difference between the vessel direction of travel (heading
corrected by drift angle relative to the earth) and the desired
track angle at the perpendicular track position. The rate term
-9-
is the inertial angular velocity normalized by the tow length
divided by the inertial velocity. The average heading command
term is used in situations with a regular current, wind, or wave
force acting on the vessel; for the Berwick Bay study, this term
is not used.
Figure 5 shows the tracks set up for the Berwick Bay down-
river passage. The first line segment holds the tow until the
tow distance from the origin of the first line exceeds the line
length. At this point, the circular arc segment is the desired
track (negative radius for counter-clockwise travel) until the
tow moves beyond the radius (or its extension) which ends thearc. The third and fourth segments control the Gow as it comes
off of the sharp turn, heading it down along the east bank until
a final straight line segment takes it through the two highway
bridges (slightly to the west of center) and through the center
of the critical railroad span opening. A small positive arc
segment then forces the tow to apply maximum rudder angle to turn
around the next river bend.
The autopilot also has the ability to change RPM, in each
track segment, holding it constant along each one. Bow thruster
control may also be incorporated through logic in the autopilot
control; for the runs with bow thruster specified, the thrusterwas used to aid the tow in turning in whatever direction the rudder
was indicating, with full thruster power applied in either direction.
Updated versions of the User's Manual and Programmer's Manual(References 1 and 2) are also being prepared under contract to
the USCG, and these provide a complete description of the version
of the simulator used to make this Berwick Bay Bridge evaluation.
(References 4 and 5.)
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THE BERWICK BAY BRIDGE PASSAGE
The Berwick Bay Bridge Passage consists of the passage of
the Atchafalaya River between Morgan City and Berwick, Louisiana.
The river describes a large figure "S" geometry as it flows south
of Drews Island through Drews Pass, curves around the first bend
at the Conrad Shipyard, flows through the Long Allen and New
Highway bridges and then the Southern Pacific Railroad Lift
Bridge, and then around the sharp bend west of Bateman Island (see
Figure 6). Figure 5 shows the bank outline and bridge positions
for the simulator model. Figures 7 and 8, from Reference 7,
show the same outline with the recommended track for best passage,
a schematic representation of the current directions, and a des-
cription of the strategy for best passage both down-river and up-river.
A detailed record of accidents involving the Berwick
Bay Bridge Passage bridges is compiled in Reference 6, and an
analysis of this data along with data on successful passage
through the bridges, available because of the records of the USCG
Vessel Traffic Service center on the east side of the railroad
bridge span opening, is presented. The concept of horsepower-
length ratio, the result of dividing total towboat engine horse-
power by overall tow length, is used in the discussion of the
casualty data to define a suggested restriction of HP/L > 3 to
preclude the probability of accident.
The VTS rules for passage through the bridges currently in effect(Reference 8) restrict tow sizes at high water stages as follows:
8. VESSEL AND TRAFFIC LIMITATIONS.
a. High Water Notification and Determination. High watervessel traffic limitations will be put in etfect and removed byNotice to Mariners. High water will be considered to exist whenthe Morgan City River Gage reads three feet mean sea level ormore for five consecutive days and is anticipated to remain at
HYDRONAUTICS, INCORPORATED
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FIGURE 6 CHART OF BERWICK BAY
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10 PORT ALLEN ROUTE
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-154-
Downstream Operation
0 Entering Berwick Bay from the Port Allen route holdthe sailing line shown and reduce speed to about halfahead.
0 Enterin Berwick Bay'from Stouts Pass cross the riverbetween5 and("and favor left descending shore.
Generally hold slow speed between(Zandowith inter-mittent use of power to stay on course and close toshore.
* At current will set tow toward right descendingshore if out too far in river.
* Cut point at Conrad ShipyardQin close to preventcurrent from catching stern or tow and rotating itout toward mid-river.
* Run between slow and half speed at to maintain steerageand control.
I Should be shaped up b 5. urrent tends to get towout-of-shape between 2and..
* At( either drive or hold half speed depending oncon itions.
* Enter highway bridge at mid span or just to theright of mid span depending on current conditions.
* Current will shift at highway bridge and operatormust expect a strong left hand draft between bridges.
* Favor right descending pier of railroad bridge to offsetcurrent and to prepare for sharp right hand bend inriver just below bridge.
0 Under some conditions with a long tow you must backand flank as soon as you clear the railroad bridgein order to line up for the passage down river.
Upriver
* In general operator can hold middle of river duringupstream approach.
*. At(slow down and line up with railroad bridge.
I Favor Berwick pier (left ascending pier) to offsetcurrent just below and between bridges.
FIGURE 8 - OPERATOR STRATEGY
-15-
three feet or more for an additional five consecutive days. Highwater limitations will be removed when the Morgan City RiverGage reads less than three feet for five consecutive days and isanticipated to remain at less than three feet for an additionalfive consecutive days.
b. High Water Vessel and Traffic Limitations. When the highwater conditions exist, the following limitations apply to vesselstransiting the navigational openings of the two highway bridgesand the railroad bridge:
(1) Towing on a hawser in either direction is prohibitedwith exception of one vessel towing another vessel in a north-bound direction.
(2) Barges and towing vessels must be arranged in tandemwith exception of one vessel towing one other vessel alongside.
(3) Towing vessels with less than 1000 horsepower shallnot tow barges with any dangerous cargo listed in paragraph 5.
(4) Southbound tow limitations:
(a) Non-integrated southbound tows without operablebow steering units shall not exceed 300 feet in length.
(b) Integrated southbound tows without operable bowsteering units shall not exceed 600 feet excluding the towboat.
(c) Southbound tows with an operable bow steeringunit shall not exceed 1180 feet including the towing vessel.
(5) Northbound tow limitations:
(a) Non-integrated northbound tows without bowsteering units shall not exceed two barges.
(b) Integrated northbound tows shall not exceed 1180feet including the towing vessel.
(c) Northbound tows with bow steering units shallnot exceed 1180 feet including the towing vessel.
(d) Northbound tows with a second towboat used onthe lead barge shall not exceed 1180 feet including the towingvessels.
A discussion of these restrictions, and also of HP/L ratio, will
be given in the conclusion of this report.
Because of the major effect of current magnitude and direction
on the bridge passage, a separate effort was made to determine an
accurate current map at a typical high water stage condition. To
develop this, Dr. L. Daggett of the U.S. Army Corp of Engineers
-16-
Waterways Experimental Station used a computer-based numerical
flow evaluation procedure to investigate the Berwick Bay Passage,
under USCG sponsorship. He made a very detailed determination
of the river and bridge co-ordinates, using large-scale Corps of
Engineers Hydrographic Survey maps. Conditions corresponding to
April 22, 1973 at 1430 were selected as the example, with a flow
of 464,000 cfs. in the main channel, and a flow into Shaffer
Bayou (see Figure 6) of 12 percent of that in the main channel.
Cross sections along the river were selected, and the surface
currents, corrected for vertical distribution of current, were
calculated at eight evenly spaced points along each of the 30
cross sections used by the simulator model. This resulted in 240
discrete current velocities and directions; the bridge co-ordinates
were then converted into the flow model co-ordinates to ensure
proper position relative to the river. The resulting current
map is shown in Figure 9. The current vectors at the new high-
way bridge (middle of the three) are 8.61 fps to 6.10 fps across
the river. In all cases, the flow directions are within three
degrees of being normal to the cross sections selected, except
immediately adjacent to the bank at the Shaffer Bayou intersection.
An "average" current number would be 7.5 fps. A detailed des-
cription of this current determination work is given in Reference
9.
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-19-
FAST-TIME AUTOPILOT RUNS
To begin the analysis, a first set of autopilot-controlled
runs were made with the following parameters:
TOWS: Hydrodynamic and propulsion data which are available
for two tows were used. The shorter, referred to in
this study as the "Tennessee", represents a two barge
fully-loaded integrated tow with a 150 foot, twin
screw, tow boat. The overall length is 745 feet. The
longer tow, referred to as the "Nashville", represents
a three barge, fully-loaded integrated tow pushed by
the same class tow boat. The overall length is 1160
feet. The towboat is a nominal 5000 horsepower vessel
with twin screws, kort nozzles, and steering and flank-
ing rudders. Additional characteristics of these tows
are:
TowboatLength overall 150 ft
Beam 42 ft
Draft 8 ft 4j in.Propeller diameter 9 ft
Displacement 1056 Short Tons
BHP 5000
Appendages Twin screws in kort nozzleswith two steering and fourflanking rudders.
Lead Barge Unit
Length overall 297.5 ft
Beam 54 ft
Depth 12 ft
Draft 9 ftDisplacement 4038 Short Tons
-20-
Trail Barge Unit
Length overall 297.5 ft
Beam 54 ft
Depth 12 ft
Draft 9 ft
Displacement 4278 Short Tons
Middle Barge Unit
Length overall 415.0 ft
Beam 54 ft
Depth 12 ft
Draft 9 ft
Displacement 6300 Short Tons
The barges and tow boat are described in more detail
in Reference 10. This reference also gives the hydro-
dynamic coefficients and maneuvering characteristics
of the "Tennessee" tow. The hydrodynamic coefficients
for the "Nashville" tow were estimated by extrapolating
the "Tennessee" coefficients on a theoretical basis.
This should be relatively accurate since all of the
hard to predict hull-propeller-rudder coefficients are
the same since the towboats are the same.
The engine characteristics of each tow are such that
approximately 5000 horsepower is required to produce
200 rpm on the two propellers. A cubic relationship
between rpm and horsepower is then assumed to model
different horsepower engines for the two tows. RMPs
of 200, 178, 156, 134 and 112 were used to produce
Horsepower/Length ratios of 6,71, 4.73, 3.18, 2.02,
and 1.18 for the shorter tow, and 4.31, 3.04, 2.04,
1.30, and 0.76 for the longer.
-21-
CURRENT: Three current conditions were selected, which were 0,
50, and 100 percent of the currents given by the WES
study.
WIND: Two wind conditions were selected, 0 and 30 knots of
wind and an angle of 245 degrees, which is a wind blow-
ing roughly from east to west. This wind direction
was selected because the fully loaded tows are relatively
unaffected by wind (vs. lightly loaded tows where the
barges present a large cross-section), with the major
effect being rotation of the tow because of the surface
area of the towboat itself at the rear of the tow.
AUTOPILOT:The autopilot tracks were selected in such a way as
to allow the tows as much distance as possible along a
single straight track before passing through the bridges
(see Figure 5). For the down-river runs, the tracks
consisted of an initial straight track to allow a
settling-down from the initial starting conditions,
a circular arc section to turn the tow as close to the
bank as possible, and then two straight tracks to head
the tow down along the east bank to a point where the
single straight track would produce a reasonable path
through all three bridges with a good alignment for
the sharp turn after the last (railroad) bridge. The
final turn was accomplished by describing a very small
circular arc, thus causing all tows to turn as hard
as possible to the right; because of the artificial
nature of this final track, no distance and heading
errors were accummulated along it, and, in reality,
the performance of the tows around this bend is un-
realistic in that flanking maneuvers would almost
always be used to negotiate it,
-22-
The three autopilot coefficients were selected to
minimize the offtrack and heading errors along the
five tracks with no current in the river. Plots of
root mean square heading angle and track error,
measured every second during the run, were made for
different values of one of the three coefficients
varied while the other two were fixed. It was found
that the minimum heading angle error and minimum
track error occurred at different values, but a rea-
sonable intermediate choice was made. A separate set
of coefficients was selected for the two tows.
In addition to measuring the root-mean-square values
(based on time with one second sampling rate) of
heading angle and track distance error, root-mean-
square command rudder angle, actual rudder angle,
and relative velocity, were also measured, Six specific
points along the fifth autopilot track, the one passing
through the bridges, were also selected at which to
measure single values of heading angle and track
error, and command rudder angle. These points were:
1500 ft before the first (old highway) bridge, 750 ft
before it, at the first bridge, at the second (new
highway) bridge, midway between the second and third
bridges, and at the third (railroad) bridge. The
values of the autopilot coefficients for the first
runs, based on no current and no wind, were as follows:
TENNESSEE
1 - 0.0 2 - 40.0 3 - 3.8 4 - 21.0
NASHVILLE
1 - 0.0 2 - 90.0 3 - 3.0 4 - 25.0
where 2 is distance error, 3 is heading error, and
4 is turn rate.
-23-
DISCUSSION OF RESULTS: The results of these first thirty runs
are shown graphically in Figure 10. In general, all
six TENNESSEE runs show good control of the tow even
under the most severe wind and current conditions;
with all five HP/L ratios over-plotted in each figure,
the composite tow tracks pass through the bridge and
make the turn to proceed down river. It can be seen
that the wind rotation causes the tracks to shift
down toward the critical east end of the railroad
bridge. The NASHVILLE runs, at no current, display
similar results, but the half-current and full-current
plots show the onset and then full presence of control
problems.
The results of the complete set of runs is shown in Figure
11, where R.M.S. track errors divided by tow length are plotted
against HP/L ratios. The TENNESSEE values are all below the
nominal bridge opening parameters of 0.177 for the 54 ft wide tow.
The effect of the wind is shown, and the worst case is the lowest
HP/L of 1.18 with full wind and current. The NASHVILLE runs show
that at half current and full current, the track errors are at and
above the nominal bridge opening parameter of 0.114. Non-
dimensionalization of the track error by tow length brings the
two tow configurations into something of a single broad trend
line. The bridge opening parameter is useful in that it provides
a measure of the magnitude of control required for a safe passage.
As a result of these first runs, it was decided to concentrate
further tests on the large tow under full current conditions, and
to consider an up-river and down-river with bow thruster case in
addition to the down-river run.
Before making a second set of autopilot runs, it was decided
to re-optimize the autopilot coefficients for the case of full
current rather than no current. Also, the concept of swepth path,
-24-
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-36-
WIND 30 I(TS AT 2450 --
NO WIND
T - TENNESSEE - 7145 FT TOWHP/L = 6.71 AT 200 RPM
4.73 1783.18 1562.02 1314
1.18 112 0.3
I0. 2.
z
i
FULL CURREN
0.1
NO CURRENT - -
00 1 2 3 41 5 6 7 8
HP/L
FIGURE lla -RESULTS OF FIRST AUTOPILOT RUNS -TENNESSEE
-37-
WIND 30 KTS AT 2450 --
NO WIND
NNASHVILLE - 1160 FT TOWHP/L 41.31 at 200 RPM
3.04 1782.04 1561.30 1340.76 112 0.3
0
0.2 Z
FULL URREN
CURRENT 0.
NOUL CURRENT
00 1 2 3 4 5 6 7 8
HPIL
FIGURE Ilb - RESULTS OF FIRST AUTOPILOT RUNS -NASHVILLE
-38-
being the off-track distance of the furthest point on the tow(front or back of tow) at any time, was introduced as a measure
of performance to act as a single optimization parameter. This
is illustrated in Figure 12, where the track and heading errors
and swept path are shown. The new autopilot coefficients for the
tows were then:
TENNESSEE:
1 - 0.0 2 - 14.0 3 - 7.2 4 - 21.0
NASHVILLE down-river
1 - 0.0 2 - 107.0 3 - 5.4 4 - 42.0
NASHVILLE up-river
1 - 0.0 2 - 110.0 3 .- 4.5 4 - 50.0
The TENNESSEE coefficients were obtained to allow comparison with
the NASHVILLE full current results. The full current paths areshown in Figure 13. Again the TENNESSEE is fine, and
the NASHVILLE has trouble. Figure 14 shows the general con-sistency of results when non-dimensionalized on length for off-track and swept path distances; the track error and swept path
results show a consistent trend, with the better heading error
results of the small tow at lower HP/L causing the largest dif-
ferences between the two tows.
Figure 15 shows, in addition to the RMS values, the in-
stantaneous track error and swept path values at the railroadbridge and the path value at the old highway bridge; these results
show the tow to be under control for all HP/L values, with the
path errors at or below the bridge opening parameter.
The same parameters for the large tow show the tow attempting
to settle on the track through the bridges, as the railroad bridge
errors are much smaller than the first bridge errors, but thewhole set of results are at or above the bridge opening parameter
on an RMS basis, and above for low HP/L at the railroad bridge
and for all HP/L at the first bridge (Figure 16),
The up-river tests were run with the autopilot tracks shownin Figure 17, where RMS values are measured only for the first
three tracks. The paths in Figure 18 show fairly good results,
with the tracks close to the west bank allowing the longest
-39-
RAILROAD BRIDGEOPENING - 318 FT
SII llI II
USCG VTS OFFICE EFFECTIVE OPENING FOR54 FT WIDE TOW- 264 FT.- C
DESIREDTRACK
TOW LENGTH, L
HEADINGERROR
RMS VALUES MEASUREDAT MID-TOW POINT EVERYSECOND DURING RUN UNTILPOINT A REACHED. TRACK
ERROR
T
INSTANTANEOUS VALUESMEASURED AT MID-TOWPOINT SWEPT
PATHR.R. BRIDGE VALUES SCORRESPONS TO MID-TOW AT POINTA S=T(T + L SIN !a1/2) ITI
S/L COMPARED TO C/2L
C/2L = 0.177 FOR TENNESSEE
= 0.114 FOR NASHVILLE
FIGURE 12 - BRIDGE OPENING AND SWEPT PATH GEOMETRY
-40-
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-42-
0-a-TENNESSEE AND NASHVILLE
8 -
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K U6 - 0.3
0Lu
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z Uzz
z w3 U..
w 3- -SWEPT PATH E
Ua.
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TRACK ERROR1-
0 00 1 2 3 4 5 6 7 8
HP/L
FIGURE 14 - RMS TRACK DISTANCE, SWEPT PATH AND HEADING ERRORSFROM SECOND SET OF AUTOPILOT RUNS
II. ._ _ - - , . , . ,_ t
-43-
TENNESSEE DOWN RIVER
RMS HEADINGANGE EROR
H 6
00
2 0.2
*-RR BRIDGE PENIN
0 3 0-.67NP/
2IUR R5STRACK DSACSETPT N EN ERROR S 0.1OR
"TENNESEE",RRB SEODSE FAUOIOTRN
-44-
9
NASHVILLE DOWNRIVER
8
7
RMS HEADING ANGLEERROR
A 6
u~ia
5.J
z
ZII 0.2
3 RMS SWEPT PATHOPENING 0
'U
RR BRIDGE u -RMS TRACK ERROR 0. -j
OLD HIGHWAY BRSWEPT PATH 10.
tA<
RR BRIDGE SWEPT PATHRR BRIDGE TRACK
<'
0 1 2 3 4 5 6 7
HP/L
FIGURE 16- TRACK DISTANCE, SWEPT PATH AND HEADING ERRORS FOR"NASHVILLE", SECOND SET OF AUTOPILOT RUNS
.1 -45-
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-47-.
straight approach to the railroad bridge as possible. The lowest
HP/L tow cannot overcome the current, and is swept away.
The numerical results of Figure 19 show paths at the bridge
above the opening parameter, but not greatly above and not increas-
ing with HP/L until the power limit is reached. Presumably some
flanking maneuver by tows going up-river would help the alignment
problem for this passage.
The final set of tests included a 10,000 pound thrust bow
thruster attached to the front of the tow. This thruster was
used to swing the tow in the direction desired by the command
rudder angle. The lower the speed of the tow through the water,
the higher the thrust obtained from this device, The paths in
Figure 20 show some improvement, and the RMS values in Figure 21
are improved, especially at the lower HP/L ratios, where speed
through the water is lower. For the up-river runs, this is also
the case as shown in Figures 22 and 23, where, in addition, the
longer times of the passages allbw the thruster to improve heading
error to the point where all swept path errors at the railroad
bridge are below the nominal bridge value.
-48-
16
NASH VILLE UP RIVER
* 14
12
RMS HEADING ERROR
o 10u
S8 -0.2uF-
z
Lu 03:u RMS SWEPT PATHui0 ..-- SWEPT PATH AT RR BRIDGE
CL Lu
RR BRIDGE OPENING LU11 0.1
RMS TRACK ERRORSWEPT PATH AT NEW -HIGHWAY BRIDGE 0
TRACK ERROR AT < 0 -RR BRIDGE
HP/L
FIGURE 19 - RESULTS OF UPRIVER AUTOPILOT RUNS
-49-
ILJL
LU
00
o LL
-50-
NASHVILLE DOWNRIVER
8- - WITHOU BOW THRUSTER
7
6 RMS HEADING ANGLE ERROR
Lua
'5<
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z 03 RMS SWEPT PATHw
u'
RR BRIDGE OPENING C.)-'
2 RMS TRACK ERROR 0.1<
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0 J00 1 2 3 54 5 6 78
HP/L
FIGURE 21 -RESULTS OF AUTOPILOT RUNS WITH BOWTHRUSTER, DOWNRIVER
-51-
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-52-
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12
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RMS HEADING ERRORLu
<8 0.2
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RR BRIDGE OPENING L
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RMS TRACK ERROR
2
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FIGURE 23 - RESULTS OF AUTOPILOT RUNS WITH BOWTHRUSTER, UPRIVER
-53-
REAL-TIME OPERATOR RUNS
The real-time operator runs were carried out with three
primary objectives:
1) To assess the ability of a human operator to follow the
autopilot tracks as followed by the autopilot using primarily
the plan position display.
2) To assess the ability of a human operator to make the passage
using both the plan position display and the visual scene
display.
3) To assess the ability of the bow thruster to aid the operator
in making the passage.
Three operators were used to make the runs.
They were:
Capt. Irvin Gros, instructor at The Harry Lundeberg School
of Seamanship, operated by the Seafarers International Union,
on Piney Point in southern Maryland. Capt. Gros has over fif-
teen years experience as a towbaot master on inland rivers, and
has made the Berwick Bay Bridge Passage numerous times. Capt.
Gros had previously spent about three hours using the USCG simu-
lator prior to the one day required to make his runs for this
study.
Mr. Peter Van Dyke, of HYDRONAUTICS, Incorporated, who has
been involved in the development of the USCG Towboat Simulator
over the past two years. He has made several hundred runs on the
simulator through the passage, but his towboat experience is
limited to one run up-river and one run downriver through the
bridges as an observer.
Mr. Eugene R, Miller, Jr., of HYDRONAUTICS, Incorporated,who has participated in the development of the Simulator through
-54-
theoretical and experimental experience. He had spent about
four hours using the simulator prior to making his runs, and alsohad made the passage as an observer on a towboat.
All operators were familiar with the current conditions used
in the simulation (Figure 5), and knew the desired track and
suggested procedures as shown in Figures 7 and 8. Capt. Gros,
during the course of his runs, remarked on the desirability of
maintaining as high an RPM as possible while making this passage,
citing several specific examples he had encountered during histime as a towboat master. This confirmed our operating procedure
during the autopilot tests, and then the manned runs, of main-
taining constant, maximum RPM on all legs of the passage.
The sequence of runs were:
Two downriver runs with the TENNESSEE, the small tow, trying
only to make the passage as cleanly as possible. Maximum
RPM of 200 used throughout, no bow thruster used.
One downriver run with the addition of the availability of
the bow thruster to aid in making the passage. RPM of 200.
Two downriver runs with the autopilot tracks drawn on the
plan position display, with the operator asked to follow
these tracks as closely as possible. RPM of 200.
Repeat of these five runs with the NASHVILLE, the long
tow.
Repeat of these five long tow runs running upriver instead
of downriver.
For the last two operators, an additional set of five runs
were made downriver with the NASHVILLE at about 157 RPM.
These are summarized in Table 1,
-55-
TABLE 1
Manned Simulation Runs
Descriptors
Operators Tows Directions B - Bow Thruster
1 - Capt. Gros T - TENNESSEE D - Downriver F - Track Following2 - P. Van Dyke N - NASHVILLE U - Upriver P - 157 RPM3 - E.R. Miller, Jr.
Run No. Operator Description Run No. Operator Description
1 1 T-D 28 2 N-U-F
2 1 T-D 29 2 N-U-F
3 1 T-D-B 30 2 N-D-P
4 1 T-D-F 31 2 N-D-P
5 1 T-D-F 32 2 N-D-B-P
6 1 N-D 33 2 N-D-F-P7 1 N-D 34 2 N-D-F-P
8 1 N-D-B 35 3 T-D
9 1 N-D-F 36 3 T-D10 1 N-D-F 37 3 T-D-B
11 1 N-U 38 3 T-D-F12 1 N-U 39 3 T-D-F
13 1 N-U-B 40 3 N-D14 1 N-U-F 41 3 N-D15 2 T-D 42 3 N-D-B
16 2 T-D 43 3 N-D-F17 2 T-D-B 44 3 N-D-F
18 2 T-D-F 45 3 N-D-P19 2 T-D-F 46 3 N-D-P20 2 N-D 47 3 N-D-B-P
21 2 N-D 48 3 N-D-F-P22 2 N-D-B 49 3 N-D-F-P23 2 N-D-F 50 3 N-U
24 2 N-D-F 51 3 N-U
25 2 N-U 52 3 N-U-B
26 2 N-U 53 3 N-U-F27 2 N-U-B 54 3 N-U-F
-56-
Insofar as possible, the tests were made with a minimum
amount of disturbance and discussion during the runs, with somediscussion between runs concerning the objectives of the next runand possible methods to be employed. For instance, Capt. Gros
on the free upriver runs tended to pass through the railroadbridge at a considerable angle (unlike the autopilot track, which
is almost normal to the opening) and then turn upriver beforepassing through the new highway bridge; this strategy was dis-cussed by the other two, less experienced operators prior to their
runs.
One error made in the test procedure was to not emphasizethat the operator should never "give up" before completing the run.
In several cases, where the tow would be out-of-shape and hitthe first bridge, the operator would not concentrate on completing
the exercise as well as he could; this tended to magnify the high
error results of-some of the runs.
The test run numbers used to plot the results are shown in
Table 1. Figures 24 through 31 show typical plan position plots
for free, free with bow thruster, and track following runs.The run number refers to the operator as described above, withthe tow identification and run type identified in the header.
The results of the track following runs are displayed in
Figures 32 and 33. In general, all operators for all four run
categories were able to maintain a heading angle error less than
that of the autopilot (Figure 33), but the track distance errorswere at or above those of the autopilot, resulting in swept path
errors both greater and less then those of the autopilot, butnever more than 15 percent different on average for any of the
-57-
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0.3 AUTOPILOT RUN
0 - MANNED RUN AVERAGE
0-J
I 0 00. 0.2
-0. 0Lu
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0 00 0 0 0 0.1 0-
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LuLUPuz * --A o.1
u<I- TENNESSEE NASHVILLE NASHVILLE NASHVILLE
DOWNRIVER DOWNRIVER DOWNRIVER UPRIVER200 RPM 200 RPM 157 RPM 200 RPM
0 18 38 9 23 43 33 Q0. 14 28 535 19 39 10 24 44 37 49 - 29 57
RUN
FIGURE 32 - RMS TRACK DISTANCE AND SWEPT PATHERROR WHILE TRACK FOLLOWING
-66-
16
0
11 TENNESSEE NASHVILLE NASHVILLE NASHVILLEDOWNRIVER DOWNRIVER DOWNRIVER UPRIVER200 RPM 200 RPM 157 RPM 200 RPM
12
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0*000 -0- - 0
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AUTOPILOT RUN AVERAGE2 --- MANNED RUN AVERAGE
014 18 38 9 23 43 33 40 14 28 53
5 19 39 10 24 44 37 49 - 29 57
RUNS
FIGURE 33 - RMS HEADING ERROR WHILE TRACK FOLLOWING
-67-
four categories of runs. The TENNESSEE tracks and the NASHVILLE
upriver tracks were more difficult to follow, while both the 200
RPM and 157 RPM downriver NASHVILLE tracks were followed very
closely in distance, with the improved heading following giving
a better RMS swept path than that of the autopilot.
The results of the free run and free run with bow thruster
are shown in Figure 34. Each of the four run categories has five
average results for the track distance error at the railroad bridge,
the swept path at the railroad bridge, and the heading angle error
at the railroad bridge. Since these errors are measured relative
to the autopilot track which passes to the left of center of the
highway bridges, but through the center of the railroad bridge, the
railroad bridge errors present the most significant measure of
the operator results. The five average results are autopilot runs,
autopilot with bow thruster, track following average, free runaverage, and free run with bow thruster average.
The TENNESSEE runs indicate, in general, track following
results comparable to autopilot results, larger errors, but
improved with use of the bow thruster (no autopilot with thruster
runs were made for this tow). The NASHVILLE downriver results
show very similar, almost identical results for all autopilot
and manned runs at 200 RPM, but a definite ability of the
operator to reduce angle errors over the autopilot, and thus
the swept path in every case except track following. The NASH-
VILLE upriver runs show the ability of the bow thruster to limit
heading error for the manned runs as in the autopilot runs. The
large heading angle variations, and thus swept path values, may
be due to the fact that Capt. Gros showed with his runs that apath quite different from that of the autopilot would produce
good results.
-68-
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-71-
The results of all of the down river runs except the track
following runs are shown in histogram format in Figure 35. In
this figure, the fraction of the number of runs with swept path
falling from 0 to 1/2 of the railroad bridge span opening, from
1/2 to full opening, and beyond full opening (collision) on either
side of the mid-span position are shown. The average values are
also presented. The effects of current seem apparent in these
results, as the tendancy to be to the left of center when the
mid-tow passes under the railroad bridge is clear. The average
values, and histogram forms themselves, indicate that for the tow
lengths and power used in the runs, no clear trend is present.
A possible reason for the relatively poor results with the shorter
tow is that these runs were the first for each operator, and were
in some sense warmup runs. The bow thruster effect is greatest
at smaller relative velocities, and inclusion of these results
helped the lower power, longer tow average swept-path error.
-72-
FRACTIONI OF RUNS
0.5Ii J
I 'TENNESSEE200 RPM
<1I I
-1 -i 0 ,} 1
OPENING FRACTION
ui -0.5 I<I NASHVILLE
,uJ 200 RPM
I I1 -i 0I 1
OPENING FRACTION
I -0 . 5 II NASHVILLEI I I157 RPM
< >I
-1 -f 0 * 1
OPEN ING FRACTION
FIGURE 35- HISTOGRAM OF SWEPT PATH ERROR AT RAILROAD BRIDGEFOR DOWN-RIVER RUNS WITH AND WITHOUT BOW THRUSTER
-73-
M/V STUD ACCIDENT SIMULATION
During the course of running the manned real-time passages,
a copy of a National Transportation Safety Board Report concern-
ing a Berwick Bay Passage collision was received (Reference 11).
On April 1, 1978, the four-barge tow of the Motor Vessel STUD
collided with the eastern fixed span of the railroad bridge over
the river. The collision knocked the span from its supporting
piers into the river but did not damage the barges. The National
Transportation Safety Board determined that the probable cause
of the accident was the failure of the master to properly align
the underpowered tow on the approach north of the Berwick Bay
bridges. Contributing to the cause were the inadequate criteria
for commencing high water limitations in the Berwick Bay Vessel
Traffice Service area, the inadequate horsepower of the STUD in
relation to the towlength for maneuvering in the existing river
conditions, and the fact that the master of the STUD did not have
up-to-date information concerning the river stage and current
velocity. It was decided that a simulation of this accident would
be possible based on the information in this report and would
demonstrate the use of the Simulator in casualty analysis.
The current conditions were estimated to be about 2.8 mph,
which is 55 percent of that used in the simulation runs. The
780 ft length of the tow was modelled using the TENNESSEE. The
accident report included an estimated path of the STUD, with times,
which was modelled using the autopilot tracks shown in Figure 36.
The first three tracks bring the tow along the estimated STUD
track; for this simulation, the RPM variation was used, as the
report indicated full RPM until point C, then half RPM to point
A, at which point recovery was attempted by the STUD. To emu-late the 690 horsepower, or 0,86 HP/L ratio, 100 RPM was taken
-74-
U)
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-75-
as full power, and 50 RPM as half. The run from times 1730 to
1740 was correctly simulated; however, the half speed portion,
reported as taking 10 minutes from 1740 to 1750 (a distance of
about 1/2 mile) took much less time. The current velocity of
3 mph would cover 1/2 a mile in 10 minutes, indicating that the
STUD was probably doing something other than maintaining half-
speed during this time.
To assess the effect of power on the potential ability of
the STUD to have avoided a collision with the bridges, which accord-
ing to the pictures in Reference 11 first occurred with the
western support of the old highway bridge, three points along the
third track, A,B, and C, were selected, and the operator tried
to navigate through the bridges by taking control from the auto-
pilot as the tow reached these points. Table I summarizes the
eighteen runs made by the three test operators. The error in-
dicator was taken as the swept path at the old highway bridge and
the railroad bridge. Two RPMs were considered; 100 to give the
0.86 HP/L ratio, and 150 for a HP/L of 3.0.
Typical plan position plots are shown in Figures 37 through
40. The early recovery point C allows alignment prior to the
bridges (Figure 37), the half point B allows fairly good recovery
(Figure 38), while the late point A results in collision with the
railroad bridge if the highway bridge is avoided, (Figure 39),
or immediately with the highway bridge (Figure 40).
The individual swept path results in Figure 41 show clearly
the decreased swept paths as the recovery point is changed, and
the general improvement in performance as power is increased,
74
-76-
TABLE 2
M/V STUD RunsOPERATORS: POSITIONS (see Figure 36)
1 - Capt. Gros A - Late2 - P. Van Dyke B - Half3 - E.R. Miller, Jr. C - Early
Run Operator RPM Position P-HB P-RRB H
1 1 100 A 0.366 0.180 H2 1 150 A 0.358 0.483 H
3 1 100 C 0.386 0.379 H
4 1 100 C 0.064 0.0935 1 150 C 0.051 0.054
6 2 100 A 0.354 0.189 H7 2 100 C 0.166 0.137
8 2 150 C 0.248 0.1699 2 150 A 0.329 0.378 H
10 2 150 B 0.168 0.089
11 2 100 B 0.148 0.086
12 2 150 B 0.075 0.070
13 2 100 B 0.119 0.066
14 3 150 A 0.348 0.420 H
15 3 100 B 0.440 0.658 H
16 3 100 B 0.385 0.417 H
17 3 150 B 0.332 0.423 H18 3 150 B 0.207 0.166
Note: P-HB - Swept path at highway bridge/tow length
P-RRB - Swept path at railroad bridge/tow length
H - H indicates bridge was struck
/-77-
l 10
• ,C
LC
IIL
'N 0w
in-
ti 0 I
m LU
le,
L)-p..
V U
LJ
A
-78-
w
IL.U
0
2K2
IU
-79-
w
LiC)
0:
u
0
0 *Q
vo FF
w
WooI-
Wo
-80-
/D I
I zx
w
-81-
-A AVERAGE OF 100 RPM RUNSAVERAGE OR 150 RPM RUNS
o 0
T 0 0S 0.1 4
<
00
* 0.3 -
a.
I--0
ulm 0.1 BRIDGE 0 0 0
OPENING 0 0
0-
0
0.4- 0
o 0- 0.3-
53 0
!- 0.2< BRIDGE
OPENING 0a. 0
0.1- 0100 RPM 150 100 RPM ISO 100 RPM 150POINT A POINT B POINT C
0II III iIII III II1 6 2914 11131516 10121718 3 47 58
RUN
FIGURE 41 - "M/V STUD" SIMULATION RESULTS, SWEPT PATHSAT BRIDGES
V ,,-i i.- ..... ... ; 7 T I
........ -. ~. - - -,z .... - ....r r
-83-
CONCLUSIONS
The overall objective of this study was to demonstrate the
use of the USCG Towboat Maneuvering Simulator in the analysis of
vessel maneuverability, navigation rules and casualty analyses.
This demonstration was provided by a simulator investigation of
the passage of a tow on the Atchafalaya River through the bridges
between Morgan City and Berwick Bay. For this specific river
passage situation a number of conclusions can be developed for
the parameters investigated. These include:
Current: The path and track errors were increased by the effects of
current (see Figure 11). The largest effects occur for conditions of
low towboat power and high current. Considering the bridge clearances
available, the 100 percent current condition represented a much more
difficult control problem than the 50 percent current case, parti-
cularly for the longer tows. When the present VTS rules on tow
size are in effect, the current velocity exceeds the 50 percent
case investigated. This is consistant with the simulation results.
Wind: The wind effects on the fully loaded tows used in this
study were small. Wind effects may be significant for empty
tows and thus should be investigated more completely.
Tow Length: For both the autopilot and real-time simulation runs,
the path errors were about proportional to tow size. For the full
current, full power cases the average swept path error at the
railroad bridge was about 0.08 of the length for both cases.
Since the comparable opening ratios are 0.114 for the long tow
and 0.177 for the short tow, the short tow has a significantly
greater margin for errors. Considering all real-time full current,
full power passages the success ratio for the small tow was 0.87
and for the long tow 0.80.
-84-
Towboat Horsepower: For both the short and long tows, the path and
track errors were not very sensitive to power until the HP/L
was reduced to about 1 or less. Considering all real-time full
current runs for the long tow, the path errors at the railroad
bridge were about the same at HP/L ratios of 4.3 and 2.0. This
is consistent with the autopilot run results. It should also be
noted that, for the long tow during full current conditions, if
the tow HP/L is reduced below about 2 it is very difficult to
slow the tow enough to flank the bend below the railroad bridge.
Bow Thrusters: The effects of the use of a bow thruster on path
error were not large for the downriver runs. This result was
consistent between the autopilot and real-time runs. The bow
thruster is most effective at low speeds through the water but under
these conditions the maneuverability is reduced. The autopilot
runs indicated a greater effect of thruster use on upriver runs,
particularly those at low HP/L ratios when water speed is low.
The analysiL of the casualty of the.M/V STUD was primarily
directed at determining if increased power would have had signi-
ficant effects. From the position at which the pilot of the M/V
STUD seems to have realized he was out-of-shape and started to
maneuver into alignment, an increase in the HP/L ratio from 0.86
to 3 would not have prevented the casualty. If the maneuvering
to properly align the tow had started 1000 ft further up river
there would have been a good chance of making it (3 successful
passages out of 4 trips) with the actual power available. From
a position starting 2000 ft further up river the passage was made
successfully two out of three attempts with the actual power and
on both attempts with a HP/L ratio of 3. Thus, in this case an
increase in the 1IP/L ratio from 0.86 to 3 increases the margin for
error at which the tow must start to maneuver into alignment by
about 1000 ft or 1-1/3 ship length.
-85-
The VTS rules for the Berwick Bay passage impose high water
limitations. For the integrated tows considered in this study
the applicable rules are:
1) Southbound (downriver) tows without bow thrusters shall not
exceed 600 ft excluding the towboat (the TENNESSEE is at this
limit), or 1180 ft including the towboat with an operable bow
thruster unit (the NASHVILLE is 1160 ft).
2) Northbound (upriver) tows shall not exceed 1180 ft including
the towing vessel.
Based on the results of this study, a number of comments about
these navigation rules can be made. There is no clear relation-
ship between the current velocity and the time high water limita-
tions are in effect. The full current case, which represents a
very high river stage, provided problems for the long tow and
in same cases for the short tows. Some form of navigation re-
gulations clearly seem justified for this case. The M/V STUD
casualty analysis indicated a current velocity of 55 percent of
the full current case just before high water limitations went
into effect. Considering the improvement in path error which
results when the current is reduced from the full to 50 percent
case, it seems reasonable that navigation limitations go into
effect for currents somewhere between the 50 and full current
conditions studied. This is consistent with the present regulations.
The present regulations are based only on tow length. The
simulations results shows that tow length is the most significant
parameter. The scope of this study did not permit a large enough
number of real-time runs to allow an absolute prediction of the
relative risk of a casualty between the short and long tow. Con-
sidering both the real-time and autopilot runs it is felt thatthe present 600 ft length limit on downriver tows is reasonable
for high water conditions.
-86-
The results using a bow thruster do not show an improvement
in path error for the long tow sufficient to make its performance
comparable to that of the short tow. Thus, it would be desirable
to further investigate the provision which allows a 1180 ft towwith a bow thruster.
For the upriver runs, the simulation results indicate thatthe 1180 ft length limit may be slightly too long. However,
the number of runs made was small and better results may have been
obtained if the two subjects with only simulator experience hadhad more practice.
In addition to the length limits, some references such as7 propose that an additional restriction on the minimum allowable
*HP/L ratio and suggest a value of 3. The simulation results do
not show much effect of HP/L ratio until the ratio is reduced to2 or below. This conclusion applies to cases in which the tow was
basically in shape for the bridge passage. The analysis of theM/V STUD casualty indicates that higher HP/L ratios can improve
the margin for error in cases in which the tow is out-of-shape.
The overall importance of HP/L ratio should be investigated further.
In addition to the specific conclusions and comments thatapply to the Berwick Bay passage, it is possible to provide more
general conclusions with respect to the use of the towboat
Maneuvering Simulator. These conclusions include:
Validation of Simulation: It is difficult to validate the results
* of towboat maneuvering simulation because of a lack of full-
scale data. However, the results of this study show good
qualitative agreement with descriptions of the passage pro-
vided by pilots. Further, the experienced pilot used for some
of the real-time runs felt that the simulator reproduced the
problems of the Berwick Bay passage very well. He was able
to adapt to the simulator displays with little trouble.
-87-
Study Techniques: It is felt that the use of a combination of
fast-time and real-time simulations is an efficient way to
study problems such as this. The scope of this study did
not allow a sufficient number of real-time runs to generate
situations with high confidence levels. Considering this
limitation, the real-time runs under pilot control gave
results, in terms of absolute path error and trends, which
* were consistent with the autopilot runs. It was noted that
the human operators tended to put more importance on heading
* error than the track-following autopilot. In future studies
the autopilot could be adjusted to show this type of per-
formance.
Simulator Use: It is felt that the results of this study do
show that the Towboat Maneuvering Simulator can be used to
assist in the solution of typical USCG problems related to
vessel maneuverability, navigation rules and casualty analysis.
-89-
REFERENCES
1. Van Dyke, P., "Towboat Maneuvering Simulator Volume 1 -Users Guide," U.S. Coast Guard, Report No. CG-D-61-79,May 1979. (NTIS - ADA 080 465)(7907-1, Vol. 1).
2. Van Dyke, Peter, "Towboat Maneuvering Simulator Volume II -
Programmer's Guide," U.S. Coast Guard Report No. CG-D-62-79, May 1979.
3. Miller, E.R., Jr., "Towboat Maneuvering Simulator VolumeIII - Theoretical Description," U.S. Coast Guard ReportNo. CG-D-63-79, May 1979. (NTIS - ADA 080 485)
4. Van Dyke, P. "Ship Maneuvering Simulator System,Volume 1 - Users Guide," HYDRONAUTICS, IncorporatedTechnical Report 8009-1, June 1980.
5. Van Dyke, P., "Ship Maneuvering Simulator System,Volume 2 - Programmer's Guide,' HYDRONAUTICS, incorporatedTechnical Report 8009-2, June 1980.
6. Dayton, R.B., "Analysis of Bridge Collision Incidents,Volume I," U.S. Coast Guard, Report No. CG-D-77-76, May1976. (NTIS - ADA 029 034)
7. Dayton, R.B., "Analysis of Bridge Collision Incidents,Volume II," U.S. Coast Guard, Report No. CG-D-118-76,May 1976. (NTIS - ADA 036 732)
8. Department of Transportation, Coast Guard, Local Notice toMariners, Special Notice 1-77, August 1, 1977. Issuedby Eigth Coast Guard District, New Orleans, La.
9. Daggett, L., "Atchafalya Currents, a Law Unto Themselves,"Private Communication to USCG, April, 1980.
10. Miller, E.R., Jr., "The Prediction of River Tow ManeuveringPerformance," USCG Report No. CG-D-32-78, June 1978.(NTIS - ADA 059 408)
11. National Transportation Safety Board, Marine Accident Report,"Coliision of M/V STUD With the Southern Pacific RailroadBridge Over the Atchafalya River, Berwick Bay, Louisiana,April 1, 1978," NTSB-MAR-80-5. (NTIS - PB80-182710)
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